The present invention finds its origin in a nickel metal hydride battery pack for particular application as a motive power source in an electric vehicle, such as an automobile. As will be evident hereafter, however, it is not just limited to such an application or environment, but may have application to series strings of batteries as are used in back-up or uninterruptable power supplies, spacecraft and the like.
In the environment of a motive power source for an electric vehicle, the battery system is subject to widely varying discharge currents and discharge rates, sometimes severe operating temperatures, deep discharges and frequent recharge events. Battery packs used in such vehicles are generally series coupled cells.
The performance and cycle life of batteries in such a pack are strong functions of operating temperature. At high temperatures, both charging efficiency and available capacity of the cells are reduced. Internal battery side reactions that produce further heating (i.e, oxygen evolution and recombination), as in an overcharge condition during an equalization or balancing process (described in more detail hereinafter), tend to be favored over those that would tend to increase battery capability. This produces even more heat that must be dissipated. Accordingly, the vehicle thermal management system for the battery pack tends to operate over longer periods to cool the batteries. This can render the vehicle unavailable for use for extended times, while also increasing energy consumption through cooling imposed when ambient and pack conditions are not optimum, and indeed when there may be very little gain in useful energy storage. In the end, cells may also have to be replaced prematurely.
One known technique for detecting an inefficient charge event is to evaluate change in battery temperature with respect to time. A change in the slope of temperature change versus time (dT/dt) signals the onset of the side reaction mentioned above. Measuring battery temperature accurately can be difficult, however, particularly in vehicles using air as the cooling medium for the thermal management system. Temperature sensors, such as thermistors, located on the battery case are influenced not only by internal battery temperature, but also by air flow and ambient air temperature. Sudden changes in ambient conditions or in the operating state of the battery pack thermal management system, for example, can result in misleading temperature signals, which can trigger a premature termination of the charging event.
Variation in the amount of energy stored in each battery at any given time gradually develops as the result of battery-to-battery performance differences, which may result from internal impedances, impurities, density of electrolytic material, age, or ambient temperature gradients across the battery pack, to name some such influences. All batteries made to the same standard at the same time from the same materials are therefore by no means identical. Small differences in cell make-up and constituent elements become exacerbated over time. Since the batteries are arranged in a pack, some may be exposed to one ambient temperature on one side of the pack, with a different temperature on the other. Areas of the pack may dissipate heat differently depending on how the batteries of the pack are exposed, or for that matter confined. These temperature gradients affect the individual cell's performance.
The capability of the battery pack, and therefore the range of the vehicle, is in large measure then determined by the battery that contains the least amount of energy. That is, in discharging such a series connected pack, the amount of useful energy depends upon the weakest cell. Accordingly, a battery that falls to a significantly lower state of charge (SOC) than the others will cause a concomitant reduction in vehicle range. Failure to equalize can also result through what is termed cell reversal, which occurs when one battery is significantly different in energy content from others in the pack, as where one cell has become fully discharged while others remain at least partially charged. Further use of the pack can cause a reverse polarity voltage in the discharged cell, causing deterioration of that battery. The ability to balance the energy, or charge, in each of the batteries improves the life of the individual batteries as well as the useful capacity of the entire pack.
Equalization of the battery pack is a process by which more charge is returned to the batteries than was removed through vehicle use or self-discharge, for instance. In a procedure where charge is being returned to all of the batteries in a pack in common during equalization, batteries that are or soon become fully charged begin oxygen recombination in an overcharge condition, and produce heat, while batteries at a lower state of charge continue to increase in capability until they also begin oxygen recombination. At that point, SOC balance is considered to have been achieved among the batteries in the pack.
This equalization, or balancing, process is typically performed over a fixed time period following normal recharge. The current that is applied to the batteries is selected to give the most effective equalization in the time being allotted for the procedure. Typically, the current is a low one passed through the battery pack, in order to bring the undercharged batteries up while minimizing the evolution of gas through electrolysis in the overcharged cells. This is because a battery at about 90% of full charge shows reduced effectiveness of charge acceptance at a high charging rate. Operating at the low current for charge equalization extends the charging process in general.
The overcharge obviously requires additional charge time, and does not significantly increase the amount of useful energy stored in the battery pack. Disadvantages of this approach include an overly-extended duration of charge, as well as additional energy consumption by the vehicle's battery thermal management system as it is caused to remove the extra heat being generated by oxygen recombination of fully-charged batteries while others in the pack reach equalization level.